Semiconductor operating circuits for low-pressure discharge lamps, typically fluorescent lamps, when operated from network voltages, typically utilize a rectifier which rectifies alternating current energy, applied through a power network. The supplied d-c is then converted to a-c in an inverter circuit, for example a push-pull half-bridge circuit as well known in the art, to provide output energy at an elevated frequency, for example between about 10-50 kHz. A resonance circuit is coupled to the inverter. The resonance circuit includes at least a resonance inductance and a resonance capacity, for example one or more capacitors. The discharge lamps have electrodes which can be heated, and at least one heater circuit is coupled to the heatable electrodes for preheating the electrodes. A semiconductor switch having its main switching path is connected in the heater circuit and, in dependence on the switched state of the semiconductor switch, the heating circuit is switched between a low-resistance and a high-resistance state. Circuits of this type are described, for example, in the referenced copending application Ser. No. 08/508,341, filed Jul. 27, 1995, Continuation of U.S. Ser. No. 08/246,738, filed May 20, 1994, Rudolph, abandoned, assigned to the assignee of the present application.
This circuit includes an inverter with a resonance circuit to operate one or more low-pressure discharge lamps having heatable electrodes. The preheating phase of the electrodes, for example electrode filaments, is terminated by a relay, or by a semiconductor switch. The relay or the switch, respectively, receive a control signal from either a voltage sensing circuit sensing a specific threshold voltage, or from a timing circuit. During the heating phase, the voltage drop across the electrode filaments of the lamp is evaluated.
When making the electrode filament, it is unavoidable that some tolerances in resistance values of the filaments result. Comparatively wide ranges of tolerances of the resistances of the filaments may arise. Even electrodes of the same type may have voltages which differ from each other across their respective electrode filaments. These variations in voltages can lead to erroneous operation; some low-pressure discharge lamps whose electrodes are comparatively cold may not be sufficiently preheated and already fire or cold-start. Furthermore, long connecting lines to the lamps can cause insufficient preheating of the electrode filaments. With long connecting lines, connecting lamps or lamp fittings or sockets to the pre-heating circuit, it is possible that even low-voltage electrode filaments simulate the impedance of warm lamp electrodes, since the impedance or resistance of the connecting lines is added to the resistance of the electrode filaments themselves. The threshold or sensing voltages are derived not from the fittings for the lamps themselves but, rather, from the remote ends of the connecting lines to the operating circuit, sometimes referred to as a ballast.